The service enables passengers to browse the web, check social media and send messages using their own smartphones, tablets and laptops at no cost.
A premium option has an introductory price tag of 14.95 US dollars (£11.74) for three hours and is being advertised as fast enough to stream music, movies and television.
The Scandinavian carrier has begun rolling out the technology on its Boeing 787-9 Dreamliner and 737 MAX aircraft.
The first connected Dreamliner aircraft was delivered last month, and more than half the fleet is due to offer free WiFi by 2020.
Norwegian’s vice president of business development Boris Bubresko said: “We’re continuously improving the in-flight customer experience and we’re delighted to be the first airline offering free WiFi for the full duration of long-haul flights.
“Millions of Norwegian customers have already enjoyed free WiFi over the skies of Europe and now long-haul passengers can continue to rely on free and high-speed internet connectivity that will enhance and personalise their journeys.
“From being the first European airline to launch free WiFi on all short-haul flights followed by free live television, Norwegian’s rollout of high quality inflight broadband services will offer business and leisure travellers even greater value at affordable fares.”